The disclosure relates to a measuring device for measuring a distance between the measuring device and a target object with the aid of optical measurement radiation.
Optical distance measuring devices are known which align a temporally modulated light beam in the direction toward a target object whose distance from the measuring device is intended to be determined. The returning light reflected or scattered from the target object aimed at is at least partly detected by the device and used for determining the distance to be measured. In this case, a typical measurement range is in a range of distances from a few centimeters up to several 100 meters.
In order to be able to measure the distance from the target object using a light beam, the light beam is temporally modulated in terms of its intensity, for example. By way of example light pulses can be emitted and a propagation time of a light pulse from emission until detection can be measured and the distance from the target object can be calculated therefrom. For this purpose, however, very short light pulses have to be emitted and very fast detection electronics have to be used in order to able to obtain sufficiently accurate measurement results. Alternatively, a light beam can be temporally periodically modulated in terms of its intensity and a phase shift between the emitted light signal and the detected light signal can be used to determine the propagation time and thus the distance from the target object. The principle of laser distance measurement is generally known by the designation “Time of Flight Ranging” for example with continuous modulation of the intensity of the light beam.
Furthermore, so-called three-dimensional (3D) cameras are known in which, in addition to an optical imaging of an object to be captured, the respective distance between a region on the surface of the object to be captured and the camera is also intended to be detected. For this purpose, the camera has an imaging optical unit that projects an image of the object sharply onto a surface of a detector arranged behind it. In this case, the detector has a multiplicity of pixels arranged in a matrix-like fashion. In this case, each of the pixels can determine image information such as, for example, a color or light intensity of the light reflected from a surface region of the target object. In addition, information about a distance between the camera and the corresponding surface region of the target object can be determined. For this purpose, the target object can be illuminated with temporally modulated laser radiation and the radiation reflected back from the target object and imaged onto the detector with the aid of an imaging optical unit can be used, by determining the time of flight, to determine spatially resolved information about distances from the respective surface regions of the target object.
However, in addition to a spatially resolving detector having a multiplicity of pixels, such a three-dimensional camera also requires an imaging optical unit in order to image each surface region of the target object precisely onto a pixel, wherein the detection signal determined from said pixel can then be used for determining the distance from the respective surface region. This requires a comparatively complicated focusing optical unit and the possibility of individual evaluation of detection signals of each of the pixels.
In contrast thereto, simple distance measuring devices are used only for determining a distance between the measuring device and the target object or a point on the target object sighted by means of a laser beam. In this case, the distance does not need to be determined in a spatially resolved manner. It generally suffices to determine an averaged distance. Such distance measuring devices are often used in handheld devices in order to determine within a room, for example, the distance from a specific location to surrounding target objects such as, for example, walls or items of furniture. In this case, a handheld distance measuring device should preferably have a construction that is as simple, robust and cost-effective as possible, and should allow simple operation.
DE 10 2006 013 290 A1 discloses a device for optical distance measurement in which a detector of a receiving unit has a plurality of light-sensitive areas which are separated from one another and which can be activated separately from one another. In this case, each of the light-sensitive areas has a photodiode, for example a PIN diode or an APD (Avalanche Photo Diode), or a CCD chip as light-sensitive element. These light-sensitive elements determine an analog detection signal corresponding to an intensity of the received light. The light-sensitive areas can be selectively activated and combined in this way to form a total detection area which can be matched as well as possible to a partial region of the detector area that is illuminated by a light source, in order in this way to improve a signal-to-noise ratio.
Since the conventional distance measuring device described uses light-sensitive elements such as e.g. PIN diodes or APDs (Avalanche Photo Diode) which provide an analog measurement signal having a high bandwidth, it may be necessary to use complicated evaluation electronics for evaluating these analog measurement signals. The light-sensitive elements operating in an analog fashion are often incompatible with a CMOS technology otherwise used in the measuring device.